The amount of traffic over radio communication networks are more or less constantly increasing. New services are developed and users tend to be online using different communication devices, for examples smartphones and tablets. More and more data is being transmitted over the radio communication networks as well. This puts increasing demands on the existing radio networks, which have finite amount of resources. Different radio networks use different technologies, e.g. Long Term Evolution, LTE, Universal Mobile Telecommunications System, UMTS, and Global System for Mobile communication, GSM. However, common for all are that they operate on so called licensed frequency bands. One operator may have an own part of the whole licensed frequency band or share a part of the whole licensed frequency band with at least one other operator.
LTE uses Orthogonal Frequency Divisional Multiplexing, OFDM in the downlink and Discrete Fourier Transform, DFT, -spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1a, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organised into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each OFDM symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks, VRB, and physical resource blocks, PRB, has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localised and distributed. In the localised resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain, thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signalling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator, CFI. The downlink subframe also contains common reference symbols, which are known to the receiving device and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 1c. 
From 3rd Generation Partnership Project, 3GPP, LTE Release-11 onwards resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel, EPDCCH. For Release-8 to Release-10 only Physical Downlink Control Channel, PDCCH, is available.
Irrespective of the technology, GSM, UMTS or LTE, the frequency resources are limited and may not be enough to meet the requirements of available resources.